206 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
When X rays were first investigated, and again when y rays were 
discovered, it was often suggested, in each case, that the radiation 
might consist of material particles. Réntgen himself proposed in 
the third of his memoirs a theory of this nature. But it was always 
felt that the difficulty of accounting for the great penetration of 
these radiations was insuperable. It seems now that this difficulty 
was quite exaggerated, and even imaginary. It does not appear out 
of place, therefore, to reconsider the position in the light of more 
recent knowledge. 
Assuming, then, that the neutral pair has great penetrating, but 
weak ionizing powers, is uninfluenced by magnetic or electric fields, 
and shows no refraction, it does so far conform to the properties of 
the y ray. And, further, if it has any moment at all, and therefore 
any external field, it may at last suffer some violent encounter which 
will resolve it into a positive and a negative, an a and a # particle. 
Of these the 8 particle would be the one possessed of much the greater 
velocity, and would appear as a secondary ray. Thus, in the neigh- 
borhood of the point of impact, an ionization would appear of much 
greater intensity than anything produced along the track of the pair 
itself. So Becquerel has found the action of the y rays on a photo- 
graphic plate to be almost entirely due to the secondary rays which 
they produce. On this view the appearance of the 8 secondary ray 
would be really a scattering of the incident ray, and this would make 
the y ray fall into line with other radiations whose secondary radia- 
tions are either scattered primary or 6 rays. 
Tf the gradual disappearance of a stream of y radiation were caused 
by collision in this way, the number disappearing in any unit of 
length of the course would be proportional to the total number in 
the stream, so that an exponential law would result. 
It appears, therefore, that ail the known properties of the y rays 
are satisfied on the hypothesis that they consist of neutral pairs. 
If the y ray is material and contains an a particle, this fact must 
be considered in reckoning the number and magnitude of the steps — 
from the atomic weight of radium to that of lead. It has been sug- 
gested to me by my colleague, Doctor Rennie, that the rayless 
changes of Ra may really be accompanied by the emission of neutral 
pairs of very small moment. This adds another unknown factor to 
the calculation. The energy involved in such emissions might be 
quite small, and, moreover, if pairs can be taken up into atoms, so 
as to form new atoms, the whole of the energy may not appear as 
heat. 
It is interesting to carry the speculation a little further and to 
observe that a pair possessing a very circumscribed field might cause 
little or no ionization, and be capable of very great penetration. Its 
